The present invention is directed to carbonless paper which can be employed in electrostatic imaging processes such as electrophotography, ionography, and the like. More specifically, in one embodiment the present invention is directed to a process which comprises incorporating into an electrostatic imaging apparatus a recording sheet comprising a support on one surface of which are situated microcapsules which comprise a shell and a core containing a color former and an oil, said microcapsules being strengthened with a polymer capable of degrading upon exposure to actinic radiation; generating an electrostatic latent image on an imaging member in the apparatus; developing the latent image with a developer; transferring the developed image to the recording sheet; and, subsequent to transfer, exposing the recording sheet to actinic radiation at a wavelength at which the polymer will degrade, thereby rendering the microcapsules subject to rupture upon application of pressure.
Carbonless paper sets generally are stacks of at least two sheets of paper wherein the application of pressure in imagewise fashion on the top sheet, typically by handwriting or typing, results in formation of a corresponding image on the underlying sheets, so that copies are formed as the image is generated on the top sheet. Carbonless paper sets typically comprise a top sheet of paper, on the bottom surface of which is coated a first composition, and a bottom sheet, on the top surface of which is coated a second composition. The first and second compositions are in contact with each other when the top and bottom sheets are placed in stack formation, and generally are of a nature such that application of pressure to the top sheet of the stack at a specified location causes interaction between the first and second compositions that results in the formation of a colored area on the bottom sheet at the location at which pressure was applied. Intermediate sheets can be located between the top and bottom sheets, wherein each intermediate sheet is coated on its top surface with the second composition and on its bottom surface with the first composition; application of pressure to the top sheet then results in the formation of a colored area at the location at which pressure was applied on each of the intermediate sheets and on the bottom sheet.
An example of a carbonless paper set is disclosed in U.S. Pat. No. 3,843,383, the disclosure of which is totally incorporated herein by reference. This patent discloses a recording sheet comprising a support having thereon a layer of color developer capable of reacting with a substantially colorless color former to form colored images. The paper set generally comprises a top sheet coated with microcapsules containing a color former solution, a bottom sheet coated with a color developer material in a binder, and, in some instances, middle sheets coated on one side with the color developer and on the other side with the color former microcapsules. Contacting a top sheet coated with color former containing microcapsules on its bottom surface with a bottom sheet coated on its top surface with a color developer and applying pressure to the paper "sandwich" thus formed results in formation of a color image. Other patents disclosing carbonless paper of this type include U.S. Pat. Nos. 2,712,507 and 2,730,456, the disclosures of which are totally incorporated herein by reference. Alternatively, as disclosed in U.S. Pat. No. 2,730,457, the disclosure of which is totally incorporated herein by reference, the color former microcapsules and the color developer of a carbonless paper can be applied to the same surface of a paper sheet. Other configurations of color former, color developer, and a pressure-releasable liquid solvent are possible, including, for example, those disclosed in U.S. Pat. No. 3,672,935, the disclosure of which is totally incorporated herein by reference. Additional patents disclosing carbonless papers and materials suitable for carbonless paper applications include U.S. Pat. Nos. 2,417,897, 3,672,935, 3,681,390, 4,202,820, 4,675,706, 3,481,759, 4,334,015, 4,372,582, 4,334,015, 2,800,457, 2,800,458, 3,418,250, 3,516,941, 4,630,079, 3,244,550, 3,672,935, 3,732,120, 3,843,383, 3,934,070, 3,481,759, 3,809,668, 4,877,767, 4,857,406, 4,853,364, 4,842,981, 4,842,976, 4,788,125, 4,772,532, and 4,710,570, the disclosures of each of which are totally incorporated herein by reference.
In some instances, users of carbonless papers may wish to pass the papers through mechanical devices that include automated paper handling systems. Such devices include printers, copiers, and duplicators for imprinting information on the carbonless sheets, as well as automatic sorting devices such as magnetic card readers and Optical Character Recognition devices for reading coded information from the carbonless sheets. All such devices contain pressure nips, including, for example, those between elements of the paper transport system such as feed belts and wheels, retard rollers, pinch rollers, and the like. When carbonless paper is passed through these devices, these elements come into contact with the surfaces of the carbonless sheets, and often become contaminated with components of the carbonless color forming coating, color developer coating, or both, which may produce a deleterious effect on the continued operation of the device. In particular, the microcapsules of the carbonless color former layer can become ruptured in a pressure nip, causing the color former solution to be deposited on one or both elements of the nip. This material may interact with other components of the carbonless coatings, or with components of other throughput materials, causing contamination and failure of the device.
For example, it may be desirable to generate images on carbonless paper sets in electrophotographic copiers and duplicators. The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrophotographic imaging process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, entails placing a uniform electrostatic charge on a photoconductive insulating layer known as a photoconductor or photoreceptor, exposing the photoreceptor to a light and shadow image to dissipate the charge on the areas of the photoreceptor exposed to the light, and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic material known as toner. The toner will normally be attracted to those areas of the photoreceptor which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This developed image may then be transferred to a substrate such as paper. The transferred image may subsequently be permanently affixed to the substrate by heat, pressure, a combination of heat and pressure, or other suitable fixing means such as solvent or overcoating treatment. Other methods for forming latent images are also known, such as ionographic methods. In ionographic imaging processes, a latent image is formed on a dielectric image receptor or electroreceptor by ion deposition, as described, for example, in U.S. Pat. Nos. 3,564,556, 3,611,419, 4,240,084, 4,569,584, 2,919,171, 4,524,371, 4,619,515, 4,463,363, 4,254,424, 4,538,163, 4,409,604, 4,408,214, 4,365,549, 4,267,556, 4,160,257, and 4,155,093, the disclosures of each of which are totally incorporated herein by reference. Generally, the process entails application of charge in an image pattern with an ionographic writing head to a dielectric receiver that retains the charged image. The image is subsequently developed with a developer capable of developing charge images.
In electrostatic imaging devices, each sheet of paper in a stack is fed sequentially into the imaging apparatus, wherein an electrostatic latent image of one polarity is formed on an imaging member. The image is then developed with a toner charged to a polarity opposite to that of the latent image, and the developed image is transferred to the paper. Transfer is frequently effected by applying an electric charge of the same polarity as the latent image (and opposite to the polarity on the toner particles) to the back of the paper sheet. The charge applied to the back of the sheet is of greater magnitude than the charge of the latent image, which results in the toner particles becoming attracted to the paper and thus transferred from the imaging member to the paper. The charge may be applied in a non-contact manner by an ion deposition device, such as a corotron, scorotron, or similar device, or by contacting the back of the sheet by a charged roller conventionally known as a bias transfer roller. When a bias transfer roller is used, the paper passes through a nip formed between the imaging member and the bias transfer roller. After transfer to the paper, the image is generally fused to the paper by conventional methods, such as application of heat, pressure, or the like. Subsequent to fusing, the stack is reassembled so that the sheets are in their proper sequence in the stack.
When carbonless paper sets are passed through copiers and duplicators, frequently a problem arises with contamination of the imaging member with tackified clusters of toner. As the carbonless paper sheets pass through the imaging device, portions of the color former composition coating the paper sheets become transferred onto the imaging member, either as a result of direct contact between the imaging member and the coated paper, or indirectly as a result of contact between the coated paper and the bias transfer roll and subsequent contact between the bias transfer roll and the imaging member, which are in intimate contact prior to and subsequent to the passage of a sheet between them. Toner particles then accumulate on areas of the imaging member where portions of the coating composition are located and become tackified, thus contaminating the imaging member. Similar difficulties with contamination can occur at other pressure nips in an imaging device, such as that formed by contact between paper feeding components, or that formed by two fuser rolls. Similar contamination problems can also occur at pressure nips in other mechanical devices with automated paper handling systems.
A potential solution to the problem of contamination of imaging devices by carbonless papers might be to provide stronger color former microcapsules that will not be broken during transport of the carbonless sheet through a duplicator. Strengthening the microcapsule walls, however, adversely affects the pressure sensitivity of the paper set, so that when pressure is applied in imagewise fashion to the top sheet, the resulting images on bottom sheets exhibit reduced image contrast. Further, strengthening microcapsule walls to a degree that still enables formation of images by application of pressure generally will somewhat reduce but not substantially reduce or eliminate machine contamination; substantial reduction or elimination of machine contamination by strengthening the color former microcapsules could require that the capsules be strengthened to a degree that any attempt to form images by application of pressure to the top sheet of the set would result in images with little contrast density or no images at all on the lower sheets.
The present invention reduces or eliminates these problems by providing carbonless paper with reversibly strengthened microcapsules containing a color former. The color former microcapsules are resistant to rupture when the carbonless paper is passed through an electrostatic imaging device, thereby reducing or eliminating contamination of the device from capsule rupture. Subsequent to passing through the electrostatic imaging device, the carbonless paper is exposed to irradiation to soften the color former microcapsules so that images with acceptable contrast density can then be formed by applying pressure to the top sheet of the set. The present invention exhibits the additional advantage of reducing or eliminating smudges of developed carbonless color caused by pressure exerted on a collated carbonless paper stack by paper feeding mechanisms such as input feeders. Since the color former microcapsules are not sensitized to pressure until after the paper has passed through the imaging apparatus, pressure exerted on the paper stack does not cause unwanted smudges on the color developer coating of the carbonless paper.
Microcapsules with shells that vary in hardness depending on exposure to radiation are known. For example, U.S. Pat. No. 4,766,037 (Watanabe et al.), the disclosure of which is totally incorporated herein by reference, discloses a photodegradable microcapsule which has a wall made of a polymer coat having an acid-decomposable bond selected from a silylether bond and a silylureido bond and which contains a compound that generates an acid upon light irradiation. The microcapsule wall changes upon irradiation by light, bringing about one of the following effects: the confined core material can be readily recovered after irradiation by light; the wall being irradiated ruptures under a very small pressure; the core becomes readily releasable upon application of heat; a liquid component around the capsule can permeate into the capsule through the wall by irradiation; or an external molten composition can readily enter into the capsule upon application of heat.
In addition, U.S. Pat. No. 4,788,125 (Davis et al.), the disclosure of which is totally incorporated herein by reference, discloses an imaging material comprising a support and a layer of photosensitive microparticles on one surface of the support, the microparticles including an image forming agent and a photosensitive composition containing a polymer which is capable of undergoing cationically initiated depolymerization and a photoinitiator including a silver halide and an organo silver salt, wherein after exposing the microparticle to radiation, the microparticles, directly or with additional processing, release the image forming agent or become permeable to a developer which reacts with the image forming agent to form a visible image. The photosensitive material is a negative working material, and it is necessary to expose the material to radiation only in the areas in which an image is desired.
Further, U.S. Pat. No. 4,782,003 (Yoshihara), the disclosure of which is totally incorporated herein by reference, discloses a photosensitive and pressure sensitive recording paper which comprises a support coated with first microcapsules each enclosing contents including a substance which has a hardness changeable corresponding to a quantity of light irradiation thereon and which includes a dye precursor, and with second microcapsules each enclosing a developer which is a thermoplastic substance. Also disclosed is a pressure developing apparatus suitable for pressure developing the photosensitive pressure sensitive recording paper. Upon application of pressure, the microcapsules variously change in hardness in accordance with the varieties of intensity of exposure.
Additionally, U.S. Pat. No. 4,587,194 (Adair et al.), the disclosure of which is totally incorporated herein by reference, discloses an imaging material and process employing photosensitive microcapsules useful in forming two or more image colors by exposure with an intensity of time modulated radiation source such as a laser. The imaging material comprises a support having at least two sets of photosensitive microcapsules on the surface. One set of microcapsules is made up of high speed microcapsules which are more sensitive to a predetermined actinic radiation than a second set of microcapsules made up of lower speed microcapsules. The microcapsules include a photosensitive composition which undergoes a change in viscosity upon exposure to actinic radiation. First and second image forming agents are respectively associated with each set of microcapsules. The imaging material can form images of two or more colors by a process which comprises imagewise exposing the imaging material to actinic radiation at a first exposure amount which is substantially quantitatively different than the second exposure amount, and subjecting the exposed imaging material to a uniform rupturing force such as pressure. U.S. Pat. Nos. 4,399,209 and 4,440,846, the disclosures of which are totally incorporated herein by reference, also disclose imaging sheets containing photosensitive microcapsules wherein images are formed by imagewise exposing the sheets to actinic radiation and subjecting the sheets to a uniform rupturing force.
U.S. Pat. No. 4,508,807 (Adair), the disclosure of which is totally incorporated herein by reference, discloses an imaging material in which images are formed by exposing a sheet having on a surface a layer of microcapsules containing a radiation sensitive internal phase, and rupturing the microcapsules, wherein images in the form of transparent windows are formed in an opaque image-receiving layer containing a light-scattering pigment by rendering the pigment transparent with the internal phase released from the ruptured microcapsules.
U.S. Pat. No. 4,873,219 (Brown et al.), the disclosure of which is totally incorporated herein by reference, discloses an improved self-contained record material having tamper resistance through an indicator quality. The self-contained record material is suitable for forming a visible mark with a focused means of pressure application, such as a stylus, needle, or pen, and which mark can then later be in part fixed by unfocused light. The self-contained record material comprises a substrate on which is coated two sets of microcapsules, only one of which is made photosensitive to change in viscosity upon exposure to actinic radiation, and the first of which are conventional microcapsules. The color formers in each of the sets of microcapsules are selected to express a different observed color. Original markings, for example, can be black (combination of orange and blue color formers). After exposure to actinic radiation desensitizing the blue color former containing capsules, subsequent markings are orange, indicating the aspect of the subsequence in time.
U.S. Pat. No. 4,742,374 (Yamamoto et al), the disclosure of which is totally incorporated herein by reference, discloses a copying apparatus including an image-illuminating device for illuminating a surface of an original having images to be reproduced, so that a light produced by the image-illuminating device is reflected by the image-bearing surface of the original. The reflected light includes rays to which a photosensitive paper is sensitive. Latent images are formed on the photosensitive paper by means of selective exposure of local portions of the photosensitive paper to the rays of the reflected light from the image-bearing surface of the original.
Although known materials and processes are suitable for their intended purposes, a need remains for electrostatic imaging processes with carbonless paper that is suitable for use in printers, copiers, and duplicators. In addition, a need exists for electrostatic imaging processes with carbonless paper that will not contaminate imaging members in printers, copiers, and duplicators. Further, there is a need for electrostatic imaging processes with carbonless paper for which the color former microcapsules will not rupture when passing through pressure nips in various mechanical devices incorporating automated paper handling systems. A need also remains for electrostatic imaging processes with carbonless paper that results in little or no machine contamination when passed through an electrostatic imaging device and that also enables formation of images of acceptable contrast density by application of pressure subsequent to passing through the electrostatic imaging device. Additionally, a need remains for a process of generating images on the sheets of a carbonless paper set wherein the machine is not contaminated and wherein the paper is capable of forming high resolution images by application of pressure subsequent to passing through the device. Further, there is a need for electrostatic imaging processes with carbonless paper that reduces or eliminates unwanted smudging on the paper as a result of pressure exerted on the paper stack in the electrostatic imaging apparatus.